1. Field of the Invention
This invention relates generally to a magnetic recording disk drive that has disks with pre-patterned servo patterns formed with a master template, and more particularly to a magnetic recording disk drive with a servo control system for positioning the read/write heads on the disk surfaces by using the servo patterns.
2. Description of the Related Art
Conventional magnetic recording hard disk drives use either horizontal recording wherein the magnetized regions that define the magnetically recorded data bits are oriented in the plane of the recording layer on the hard disks, or perpendicular recording wherein the magnetized regions are oriented perpendicular to the plane of the recording layer. Each recording surface on the disks is a continuous layer of magnetic material that becomes formed into concentric data tracks containing the magnetically recorded data bits when the recording head writes on the magnetic material. Each disk surface also includes a fixed, pre-recorded pattern of servo sectors that cannot be written over by the recording heads and that are used to position the heads to the desired data tracks and maintain the heads on the data tracks.
The conventional method of generating the pattern of servo sectors is by “servo-writing” the pattern on a track-by-track basis, either with a special write head and servo-writer or with the production recording head in the disk drive. Because this is a time-consuming and therefore expensive process, other methods for generating the servo patterns have been proposed.
In contact magnetic duplication or transfer (CMT), sometimes referred to as magnetic printing or magnetic lithography (ML), a “master” template is used that contains regions or islands of soft (low-coercivity) magnetic material in a pattern corresponding to the servo pattern that is to be transferred to the disk. The CMT master template is typically a rigid substrate or a rigid substrate with a plastic film formed on it, as described in U.S. Pat. Nos. 6,347,016 B1 and 6,433,944 B1; and by Ishida, T. et al., “Magnetic Printing Technology-Application to HDD”, IEEE Transactions on Magnetics, Vol 39, No. 2, March 2003, pp 628-632. U.S. Pat. No. 6,791,774 B1, assigned to the same assignee as this application, describes a CMT template and process for forming servo patterns in perpendicular magnetic recording disks. Magnetic lithography (ML) using a flexible master template is described in U.S. Pat. No. 6,798,590 B2, assigned to the same assignee as this application, and by Bandic et al., “Magnetic lithography for servowriting applications using flexible magnetic masks nanofabricated on plastic substrates”, Microsystems Technology, DOI 10.1007/s00542-006-0287-8.
The CMT process for forming servo patterns is applicable not only to conventional “continuous” magnetic media wherein the concentric data tracks are formed in the continuous layer of magnetic material by the recording heads, but also to “discrete track” media. In this type of media, as described for example in U.S. Pat. No. 4,912,585, each data track consists of continuous magnetic material, but the individual data tracks are separated by nonmagnetic guard bands. The CMT process may be used to form not only the servo patterns but also the discrete tracks.
Patterned magnetic media has been proposed to replace conventional continuous magnetic media to increase the data storage density in disk drives. In patterned media the magnetic material on the disk surface is patterned into small isolated data islands such that there is a single magnetic domain in each island or “bit”. To produce the required magnetic isolation of the patterned data islands, the magnetic moment of the regions between the islands must be destroyed or substantially reduced so as to render these regions essentially nonmagnetic. Alternatively, the patterned media may be fabricated so that that there is no magnetic material in the regions between the islands. Patterned media can be produced by replication from a master template via nanoimprinting. The nanoimprinting process forms not only the isolated data islands in the data tracks, but also the servo patterns. In nanoimprinting a master mold or template replicates a topographic pattern onto a polymeric resist coating on the disk substrate, followed by sputter deposition of magnetic material over the pattern. Nanoimprinting of patterned media is described by Bandic et al., “Patterned magnetic media: impact of nanoscale patterning on hard disk drives”, Solid State Technology S7+Suppl. S, SEP 2006; and by Terris et al., “TOPICAL REVIEW: Nanofabricated and self-assembled magnetic structures as data storage media”, J. Phys. D: Appl. Phys. 38 (2005) R199-R222.
In hard disk drives, the servo pattern on the back surface of the disks is not identical to, but is rather the mirror image of, the servo pattern on the front surface of the disks. Thus, to form servo patterns by either CMT or nanoimprinting, it is necessary to fabricate two master templates, one for the front surfaces of the disks and one for the back surfaces of the disks. This doubles the time and cost to fabricate the master templates. In the case of nanoimprinting, the master template can be very expensive and require several days to fabricate because it is typically generated by relatively costly and slow e-beam lithography equipment.
Pending application Ser. No. 11/740,289, filed Apr. 26, 2007 and assigned to the same assignee as this application, describes a disk drive with disks that have identical pre-patterned servo patterns on the front and back disk surfaces. The servo sectors on the two disk surfaces of each disk form identical patterns of angularly spaced arcuate-shaped lines that extend generally radially across the data tracks. The arcuate-shaped lines on one surface, e.g., the front surface, generally replicate the path of the recording head as it is moved across the data tracks by the rotary actuator, so that there is a constant sampling rate of the servo sectors on the front surface regardless of radial position of the head. However, the arcuate-shaped lines on the other surface, i.e., the back surface, do not replicate the path of the recording head so the servo sampling rate is not constant but varies with radial position of the head. Thus when the servo control system is operating from servo sectors on the back surface, the servo control processor calculates a timing adjustment from an estimate of the radial position of the head. This timing adjustment is then used to adjust the time to open a time window to allow detection of the servo sectors on the back surface.
What is needed is a magnetic recording disk drive that has disks with identical servo patterns on the front and back disk surfaces, and a disk drive with a servo control system that can operate with the identical servo patterns but that does not require the calculation of a timing adjustment.